There are two categories of sealed lead-acid cell. These are the non-recombining or partially recombining type, such as those manufactured by Sonnenschein and by Crompton-Parkinson Ltd, and the fully recombining types, as manufactured by the General Electric Company and by the Gates Rubber Company. The fully recombining types are also produced in the UK under licence by Chloride Energy Ltd under the trade name Cyclon.

Particularly towards the end of charge and when being overcharged, the sulphuric acid electrolyte in lead-acid batteries undergoes electrolysis to produce hydrogen and oxygen.

Consequently, in service, the electrolyte level drops and the concentration of sulphuric acid increases, both of which are deleterious to battery performance and, unless attended to by periodic topping up with distilled water, will lead to the eventual destruction of the battery.

Aware of this danger manufacturers recommend a periodic topping up of the electrolyte to the prescribed mark with distilled water. The need for regular topping up has in the past limited the applications in which lead- acid batteries can be used. Manufacturers have adopted two methods of avoiding the need to top up lead-acid batteries:

1. The development of non-recombining or partially recombining batteries in which, by attention to battery design (new lead alloys for grids, etc.) and by using more sophisticated battery charging methods, gassing is reduced to a Aninimum and topping up is avoided.

2. The development of fully recombining types of battenes in which any hydrogen and oxygen produced by gassing is fully recombined to water, thereby avoiding loss of electrolyte volume.

Both methods have been used to produce a range of non-spill either partially or fully recombining sealed lead-acid batteries which are now finding an everincreasing range of applications for this type of battery.

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